An electrohydraulic gear control device has been known from DE 10 2005 012 586 A1. The electrohydraulic gear control device is designed with an electric gear control unit and several pressure control valves, which are electrically controllable thereby, as well as several pressure regulating valves and switching valves, acted on by a hydraulic pilot pressure supplied by pressure control valves, such that gear switching elements may be controlled by way of the control pressure required for representing the respective operating state. Moreover, an emergency gear shifting valve is provided, which is located in the first shift position, when the gear control unit is activated, in which the respective switching elements connectable to the force flow for representing the required operating state of the transmission may be correspondingly connected and is located in the second shift position, when the gear control unit is de-activated, in which the respective switching elements connectable for representing an emergency transmission associated with the transmission area, are controllable by way of the control pressure required for this purpose. In addition, a selective switching valve is provided which interacts with the emergency gear shifting valve such that the emergency transmission associated with the transmission area is engaged.
The above-described electrohydraulic gear control unit is provided to operate 6-gear transmissions in which the different forward drive and reverse drive gear ratios are respectively represented by exposing two switching elements to pressure each time. Furthermore, by simultaneously connecting two switching elements, an emergency transmission is also provided, which may be engaged via an entirely hydraulic connection in case of a failure of the electric gear control unit, during which the corresponding valve devices are not energized.
It is the object of the present invention to further develop an electrohydraulic control device known from prior art, such that vehicle availability is guaranteed in case of a failure of an electric gear control unit in a simple and cost-effective manner even with an 8-speed transmission.
According to the present invention, this object is attained by way of an electrohydraulic control device of an 8-speed transmission